1. Field of the Invention
The present invention relates to a novel method and apparatus for color radiography applied to medical diagnosis or various kinds of non-destructive inspections, and color light emission sheet therefor.
2. Description of the Related Art
In radiography used for medical diagnosis or industrial non-destructive inspection, it is usual to use a combination of an X-ray film and an intensifying screen to enhance sensitivity of a radiography system. In the radiography, light converted into visible light by irradiating X-rays transmitted through a subject to be inspected on the intensifying screen reduces for instance silver grains on a monochrome X-ray film to blacken, thereby obtaining a transmission image of the subject.
A radiation intensifying screen used in radiography or the like is generally constituted of a support consisting of paper board or plastics, a phosphor layer having a light emission peak corresponding to the X-ray film, and a protective film for protecting the phosphor layer, laminated in this order. Recently, in addition, there is a method where with a light detecting element such as a CCD camera or the like as an imaging system to do without the X-ray film, difference of an amount of transmission of the radiation being digitally detected.
X-radiography for medical diagnosis is applied to various parts of a human body to find out various kinds of foci. In recent years, in order to improve detection sensitivity, a higher contrast X-ray film is main stream. For instance, in mammography due to X-rays (mammography, hereafter), calcification and abnormal soft tissue in a mamma in which difference of X-ray absorption is very scarce have to be radiographed with high resolution and appropriate contrast. To this end, an X-ray tube having a Mo target generating X-rays of approximately 30 kV is used and, in addition, a high contrast X-ray film being used.
In the aforementioned X-radiography, energy of irradiated X-rays and an irradiation period have to be optimized according to the subject, thereby a radiogram of an appropriate film density being obtained. Conditions for radiography are determined further based on a dynamic range (latitude) of the X-ray film, parts to be radiographed of a human body that is a subject and individual difference.
Optimization of the radiographing conditions necessitates a lot of experiences to result in depending on individual technician""s skill. Accordingly, depending on the technician""s skill, the conditions may deviate from the optimum ones to result in poor X-ray exposure (black radiograph) or excessive X-ray exposure (blank radiograph). In particular, when an X-ray film of high contrast is used, the range of the optimum conditions is very narrow to be likely to result in the poor exposure or excessive exposure.
That is, the contrast characteristics of an existing X-ray film can be understood from a characteristic curve of a film as shown in FIG. 13. In FIG. 13, ordinate denotes film density when the film is exposed, abscissa denoting logarithmic value of the exposure (relative value). The characteristic curve of the film can be divided into three portions based on its shape. A curve portion A of relatively low exposure is called a leg region and corresponds to a low film density portion of a radiograph to result in a very low contrast image or no contrast image. A curve portion C of relatively high exposure is called a shoulder region. There is an upper limit in film density. Accordingly, exposure variation in the C region does not cause variation in contrast.
The highest contrast region B is located interposed between the aforementioned leg region and the shoulder region. The characteristic curve in the region B has a relatively straight and large gradient. The characteristic curve of the X-ray film is determined dependent on parameters such as a grain diameter of silver compound in an emulsion and a thickness thereof. Accordingly, by controlling these parameters, the films different. in sensitivity and contrast characteristics can be obtained. The high contrast X-ray film is one the gradient of which is large in the region B of the characteristic curve.
The densities of the leg and shoulder regions of the characteristic curve are approximately the same for all films. Accordingly, the larger gradient of the characteristic curve causes a narrower range of exposure (latitude) in the region B. In radiographing, the X-ray exposure is preferable to be set at just midway of the region B. However, when an X-ray film of particularly narrow latitude is used, a slight deviation of the conditions causes an image of an inappropriate density. In the existing X-ray film, a width of latitude is approximately one to two digits.
Furthermore, as in the case of the target subjects being blood and tissue, when element compositions of the target subjects are different, taking X-ray energy to be used and the thickness of the subject into consideration, an irradiation period (exposure period) has to be determined based on much experience. When, as in the case of normal tissue and abnormal tissue such as cancer tissue, the element compositions are approximately the same but the densities are different, the situation is also the same. In setting such conditions, the skill of the technician affects largely. In particular, in recent medical diagnosis, as in the case of early findings of cancer for instance, there is a strong demand for a correct detection of an extremely small abnormal tissue. However, a slight deviation of the radiographing condition may cause a radiograph of an inappropriate film density.
Such problems, without restricting to the radiography for medical diagnosis, also similarly occur in the industrial non-destructive inspection. For instance, when the target subjects are aluminum and iron, due to density difference thereof, the optimum conditions for radiographing are naturally different. In addition to this, the thickness of the target subject has to be considered. Furthermore, when there are contained a plurality of substances as in composite material, many radiographs have to be taken while changing the irradiation condition, handling inconveniences causing many problems.
In the existing radiography, it is general to obtain, with the monochrome X-ray film :as mentioned above, a radiograph of a target subject as a monochrome gray-scale image. In the monochrome gray-scale image, it is difficult to draw information out of a slight density change. To overcome such difficulties, there is proposed color radiography (cf. Japanese Patent SHO 48-6157 Official Gazette and Japanese Patent SHO 48-12676 Official Gazette). In the above color radiography, a fluorescent screen (or intensifying screen) furnished with a plurality of line spectra by means of two or more kinds of phosphors is used, thereby the respective color sensitive layers of color film being independently sensitized.
According to the color radiography, a radiograph in which a color changes in accordance with the difference of an amount of X-rays (color radiograph) can be obtained. In the obtained color radiograph, the low exposure portion is colored in red, as the exposure increases a green color starts to mingle with red, a further increase of exposure causing blue to mingle with red and green. A still further increase of the exposure results in white.
However, how hard trying to draw information only out of color variation on the color radiograph, for instance in the portion where much X-ray is exposed, as a result of addition of green and blue to red, the color becomes whitish to be rather difficult in drawing out the information. Furthermore, in the lower exposure portion, there is no difference from the existing monochrome radiograph until the red color component saturates. Accordingly, for the part of lower contrast in comparison with the existing monochrome radiograph, it is difficult to draw out the information.
As mentioned above, in the existing radiography, in particular when a high contrast X-ray film of which gradient in the B region of the characteristic curve is made larger is employed, a slight deviation of the radiographing conditions results in a radiograph of an inappropriate density. Furthermore, since an amount of X-ray transmission depends on a specific gravity and density of a target subject, when radiographing parts where there are substances of different specific gravity or parts where there are the same substances of different densities, the radiographing conditioning is very difficult to set. From these too, a radiograph of an appropriate density can not be obtained.
By contrast, the existing color radiography obtains a color radiograph in which in accordance only with the difference of the amount of X-rays, a color is varied. It is difficult to draw information only out of color variation on a color radiograph. Even if there is a lot of information on the radiograph, it can not be effectively utilized. Furthermore, depending on the case, the information can be drawn out with much difficulty than in the ordinary monochrome radiograph.
From the above, there is a strong demand for a radiography system that with for instance the contrast of a radiograph increased, while preventing poor exposure or excessive exposure due to a slight deviation of the radiographing condition from occurring, further enables to utilize effectively much of the obtained information. That is, a radiography system that in addition to obtaining radiographs of appropriate film density under a relatively broad condition, enables to obtain effectively a great deal of information from the obtained radiograph is demanded. Alleviation of condition setting during radiographing can not only prevent miss shots during radiographing but also largely contribute in increasing inspection accuracy.
Accordingly, an object of the present invention is to provide a system for radiography, that is, a method and apparatus for color radiography, in which even when for instance a contrast of a radiograph is increased, under various conditions a radiograph of appropriate film density can be obtained. Another object of the present invention is to provide a method and apparatus for color radiography that enables to obtain assuredly and effectively a lot of information by radiographing one time. Still another object of the present invention is to provide a color light emission sheet used for such radiography system.
The method of color radiography of the present invention comprises a step of irradiating radiation on a subject, a step of irradiating the transmitted radiation on a phosphor, and a step of separating the light into the respective colors to detect. In the step of irradiating the transmitted radiation on a phosphor, the radiation transmitted through the subject is irradiated on the phosphor that emits in a plurality of colors due to the radiation, ratios of light emissions of the plurality of colors to radiation of the same intensity being different. The step of separating the light into the respective colors to detect separates the light emitted in a plurality of colors from the phosphor under the irradiation of the radiation into the respective colors to detect.
In the present method for color radiography, as a specific means for differentiating the ratios of light emissions of a plurality of colors, a method using for instance a phosphor can be cited. The phosphor comprises a primary emission component, and at least one secondary emission component. The primary emission component corresponds to one emission color in the visible light region. The secondary emission component has an emission color different from that of the primary emission component and is different in a ratio of light emission to the radiation of the same intensity from that of the primary emission component. Furthermore, the light emitted from the phosphor can be allowed to transmit through a color filter unit to adjust the ratios of the light emissions of a plurality of colors.
In the present method of color radiography, the step of separating the light to detect can be implemented as follows. That is, after collectively imaging the light emitted in a plurality of colors from the phosphor, the respective color signals corresponding to the light emissions of a plurality of colors are separated from the image to detect. Alternatively, the step of separating the light to detect can be implemented by separating the light emitted in a plurality of colors into the respective colors with a light detection element to detect.
Furthermore, the present method for color radiography can be configured as follows. That is, with at least two kinds of phosphors each containing as a primary component an element different in K absorption edge from the other, a substance having a K absorption edge intermediate between the K absorption edges of the aforementioned elements is detected. Such method of color radiography is particularly effective in angiography or the like.
An apparatus for color radiography of the present invention comprises a radiation:source irradiating radiation to a subject, color light emission means, and means for separating/detecting. The color light emission means has a phosphor that upon irradiating the radiation transmitted through the subject, emits in a plurality of colors due to the radiation, ratios of the light emissions of the plurality of colors to the radiation of the same intensity being different. The means for separating/detecting separates the light emitted in the plurality of colors from the phosphor based on the irradiation of the radiation in to the respective colors to detect.
In an apparatus for color radiography of the present invention, for light detection means, for instance, a color X-ray film, a color camera, and a combination of color separating means and a plurality of monochrome cameras can be used. The color X-ray film converts collectively the light emitted in a plurality of colors from the phosphor into a color image. The color camera detects collectively the light emitted in a plurality of colors. The color separating means separates the light emissions of a plurality of colors. The plurality of monochrome cameras detects the light emissions of the separated respective colors.
In the present method and apparatus for color radiography (hereafter, color radiography system), a phosphor emitting in a plurality of colors under the irradiation of radiation enables to have different information for each color, furthermore the information contained in the respective colors being separated into the respective colors to detect. Thereby, the information contained in the respective color signals can be effectively and assuredly obtained. In addition, through acquisition of a plurality of image information having different sensitivity characteristics for the respective colors, the dynamic range during radiographing can be broadened.
In the present invention; a color light emission sheet containing a phosphor having for instance a plurality of emission wavelength regions in the visible light region can be used. That is, a phosphor having an emission spectrum corresponding to at least two emission colors among blue emission, green emission and red emission can be used for the above sheet. Now, light emitted in a plurality of colors from such color light emission sheet is collectively converted into an image on a color film. When the ratios (brightness) of the light emissions of the plurality of colors to the radiation of the same intensity are different, the characteristic curve as shown for instance in FIG. 13 can be plurally obtained in different exposure ranges.
FIG. 1 shows one example of characteristic curves obtained from color films exposed to the light emitted from a color light emission sheet when X-rays are irradiated thereon while varying an amount of X-ray irradiation. The color light emission sheet comprises a phosphor of which red light emission as the primary light emission component is 60%, green light emission as a first secondary light emission component 30%, and blue light emission as a second secondary light emission component 10%. When the characteristic curve between film density and exposure for each of three colors is the same as shown in FIG. 13, as shown in FIG. 1, a plurality of characteristic curves different in exposure range can be obtained. From FIG. 1, it is found that when the red light emission has saturated the green and blue ones have not, when the green one has saturated the blue one has not.
By obtaining a plurality of characteristic curves, a range of exposure (latitude) for an appropriate range of film density required in radiography can be largely expanded in comparison with the existing case of one characteristic curve (FIG. 13). If an appropriate range of film density is 0.5 to 3.5, a relative exposure corresponding to the range of film density is approximately 1 in FIG. 13, by contrast approximately 1.8 in FIG. 1. Since the relative exposure is a logarithmic value, the above value means an expansion of the range of exposure to approximately 6.3 times (=101.8/101).
That is, according to the present color radiography system, the dynamic range in taking radiographs can be largely broadened. The situation is identical even when, instead of the color film, a light detecting element such as a CCD camera or the like is employed. Accordingly, even if the system conditions or the radiographing conditions are a little bit deviated from the appropriate range, an image of a density appropriate for medical diagnosis or non-destructive inspection can be obtained. This largely contributes in suppressing failure due to poor exposure or excessive exposure during radiographing.
In the present color radiography system, much information based on a plurality of characteristic curves is separated from the aforementioned image information into the respective color signals to detect. Accordingly, much information contained in the respective color signals can be effectively and assuredly obtained. In other words, a plurality of image information having a sensitivity characteristic different for each color can be obtained. Accordingly, by taking the advantage of such plurality of image information to carry out medical diagnosis or non-destructive inspection, medical diagnosis ability and accuracy in non-destructive inspection can be greatly improved. That is, the dynamic range in the radiography for medical diagnosis or for non-destructive inspection can be expanded.
The color light emission sheet of the present invention comprises a sheet base, and a phosphor layer configured in a single layer that is disposed on the sheet base and contains a phosphor. The phosphor has a primary emission component emitting primarily to radiation and at least one secondary emission component. Of the secondary emission component, an emission color is different from that of the primary emission component and a ratio of light emission to the radiation of the same intensity is different from that of the primary emission component. Here, the ratios of the light emissions of the primary emission component and the secondary emission component are adjusted according to the dynamic range of the radiographing system.
In the color light emission sheet of the present invention, for the phosphors constituting the phosphor layer, the following can be preferably used. For instance, a europium activated gadolinium oxysulfide phosphor and a europium activated yttrium oxysulfide phosphor can be preferably used, the ratios of the light emissions of the primary emission component and the secondary emission component being adjusted through an amount of europium activator. A terbium activated gadolinium oxysulfide phosphor in which the ratios of the light emissions of the primary emission component and the secondary emission component are adjusted through an amount of terbium activator can be preferably employed. In addition, a calcium tungstate phosphor in which part of calcium is replaced by magnesium to adjust the ratios of the light emissions of the primary emission component and the secondary emission component can be preferably employed.